1. Field of the Invention
The present invention relates generally to optical devices such as lasers, and fiber optic data transmission systems employing the same, and particularly to a novel wavelength-locked loop servo-control circuit for optimizing performance of optical signal processing equipment, especially equipment employed in wavelength division multiplexing (WDM) systems and dense wavelength division multiplexing (DWDM) systems.
2. Description of the Prior Art
Wavelength Division Multiplexing (WDM) and Dense Wavelength Division Multiplexing (DWDM) are light-wave application technologies that enable multiple wavelengths (colors of light) to be paralleled into the same optical fiber with each wavelength potentially assigned its own data diagnostics. Currently, WDM and DWDM products combine many different data links over a single pair of optical fibers by re-modulating the data onto a set of lasers, which are tuned to a very specific wavelength (within 0.8 mm tolerance, following industry standards). On current products, up to 32 wavelengths of light can be combined over a single fiber link with more wavelengths contemplated for future applications. The wavelengths are combined by passing light through a series of thin film interference filters, which consist of multi-layer coatings on a glass substrate, pigtailed with optical fibers. The filters combine multiple wavelengths into a single fiber path, and also separate them again at the far end of the multiplexed link. Filters may also be used at intermediate points to add or drop wavelength channels from the optical network.
A key factor in determining the ultimate detector sensitivity and bit error rate in fiber optic wavelength multiplexing systems is spectral interference at the wavelength of interest caused by overlap between adjacent wavelength bands. This form of optical crosstalk is especially important in DWDM systems, where the wavelength spacing is currently standardized at 0.8 nm and may be reduced to as little as 0.4 nm or less on next generation systems. In addition to applications in the design of WDM networking equipment, this approach is very valuable in the design of WDM optical test equipment. Weak signals at the wavelength of interest may also be nested in broadband background noise, which also limits their detection by decreasing ambient signal-to-noise ratio. This problem is additionally pertinent to spectroscopy and other forms of optical signal processing equipment.
It would thus be highly desirable to provide a system and method for automatically improving detection sensitivity in WDM and DWDM systems.
One technique which may potentially improve the detection limit by more than an order of magnitude involves measuring either the first or second derivative of the optical transmission curve with respect to wavelength. The derivative output signal is directly proportional to the optical loss (for example, due to absorption, impurities, or crosstalk) in the wavelength communication channel.
It would thus be highly desirable to provide an apparatus and method for implementing derivative measurement techniques in a practical apparatus compatible with existing WDM and DWDM network equipment.